Safety valves are used in safety-oriented fields, especially in the chemical industry and in power plants. They are used in emergencies and must always operate reliably. An actuator, which comprises a pneumatic actuating drive and a drive rod, causes the safety valve to perform translational or rotational movement.
It is known that the actuator can be tested by a diagnostic device. In such cases, a closing function and an opening function, for example, are checked. For example, a distance-time diagram of the actuation performed by the actuator, an end position, or some other variable can be recorded. The actuator is connected to a position controller, which is mounted on the actuator. The position controller acquires data which can serve diagnostic purposes.
The position controller is supplied by, for example, a 24-V power line. For safety reasons, a valve closing function is achieved by turning off the voltage supply completely. It is disadvantageous in this case that, because of the absence of an operating voltage, it is impossible to obtain any diagnostic data for a microprocessor.
Although it is possible to use batteries or additional power lines, this is not desirable in areas where there is the danger of explosion.
The so-called “live zero” method is known. Here, an opening function and a closing function of the valve are achieved by the use of two different current levels, namely, one of 4 mA and a second of 20 mA. A closing function is realized at 4 mA, and an opening function is realized at 20 mA. The weak 4-mA current, however, is also used to continue to supply a microprocessor with energy, so that, even if the power is turned off, diagnostic data can still be obtained.
A position controller for an actuator of a safety valve is known from WO 99/21066; this controller has a diagnostic unit. The diagnostic unit is intended to make it possible to verify fault-free operation. The position controller comprises a control unit, connected to the actuator; a microprocessor unit connected to the control unit; and a communications interface connected to the microprocessor for communicating with a display unit. The position controller is supplied with 24 V. When the voltage falls to zero, the actuator is actuated by the force of a spring, i.e., moved into a safety position. The actuator is activated by the release of the pressure. The microprocessor unit conducts diagnoses at certain intervals. During normal operation, the microprocessor conducts these diagnostic tests of the actuator at certain intervals. When an emergency function is initiated, a signal is sent directly from a separate safety system. The controller also includes a nozzle/baffle system for the emergency closing function and a separate nozzle/baffle system for the diagnostic function. This design suffers from the disadvantage that a defect in the nozzle/baffle system is not detected by the diagnosis.
It is a disadvantage, however, that, when this voltage drop occurs, the microprocessor does not continue to be supplied with energy and is thus unable to operate. The safety function therefore has a higher execution priority than the diagnostic function, which cannot be conducted during the emergency function.
DE 44 29 401 C2 discloses a pressure medium-operated actuating drive with a pneumatic control stage based on the principle of an I/P converter. A switch element is provided in the electrical drive path between an electronic circuit and the control stage. When a malfunction occurs, this switch element cuts off the electrical drive of the control stage, so that this stage can then assume a safety position and the pressure medium-operated actuating drive can be vented. The electronic circuit is supplied over a line carrying 4-20 mA according to the so-called “live zero” principle. That is, the current for venting the actuating drive is not zero but rather 4 mA. As a result, the electronic circuit can still be supplied with energy. The switch is a relay switch. The relay is controlled over a line separate from the 4-20-mA line. This additional line, however, can lead to problems in rooms subject to the danger of explosions. In any case, it represents a considerable extra expense.
An emergency shut-off system with an emergency valve or safety valve is also known from EP 1 161 636 B1. In this system, an electrical circuit is installed between a magnetic valve control unit and an emergency control device. This circuit, which consists of resistors, a capacitor, and a semiconductor component, has an impedance-converting function. The goal of converting the impedance is to adapt the voltage output of the emergency shut-off control to the current input of the digital valve control. In this solution, the devices for diagnosis and for shutting off the actuating element are set up separately. Diagnoses cannot be conducted during the execution of the emergency function in this case either.